1. Field of the Invention
The present invention relates to an image forming apparatus capable of performing a color printing. More particularly, it relates to an image forming apparatus which is so configured as to transfer toner images, which are formed on a plurality of photosensitive drums corresponding to respective colors, sequentially to an image transferred member such as an intermediate transferring belt in superimposition to form a color image, and then transfer the color image to a sheet.
2. Description of the Related Art
A so-called tandem (in-line) type image forming apparatus capable of performing a color printing includes a plurality of photosensitive drums (image bearing members) which are sequentially arranged so as to correspond to toners of respective colors (generally, four kinds of toners including cyan, magenta, yellow and black), and an intermediate transferring belt so provided as to come in contact with peripheral surfaces of the photosensitive drums. Toner images formed respectively on the peripheral surfaces of the photosensitive drums are transferred to the intermediate transferring belt in superimposition while the intermediate transferring belt is rotated. This allows a color image to be formed on a surface of the intermediate transferring belt (primary transfer). The color image formed on the intermediate transferring belt is transferred to a sheet (secondary transfer). The sheet on which the color image is printed by the secondary transfer is discharged to outside after a predetermined fixing processing is applied.
Each of the photosensitive drums faces a respective transferring roller through the intermediate transferring belt. An electric charge having a polarity opposite to a polarity of the toner image is applied to the intermediate transferring belt through the transferring roller. Accordingly, the toner image formed on the photosensitive drum is electrically peeled off and transferred onto the intermediate transferring belt.
Meanwhile, since a photosensitive drum for black toners is provided on a downstream-most side in a rotational direction of the intermediate transferring belt, the black toners are generally superimposed onto the intermediate transferring belt after toners of other colors are superimposed. As described above, since the black toners are superimposed on the color toners on the intermediate transferring belt, the black toners are layered directly on the sheet with toners of other colors superimposed on the black toner layer when the color image is transferred from the intermediate transferring belt to the sheet. Such superimposition of toners of respective colors achieves a favorable image quality of the color image.
Further, in the case where a monochromatic printing using only black toners is performed in a color image forming apparatus, and if a photosensitive drum for black toners is arranged on an upstream side of the intermediate transferring belt, it is necessary that a transfer position on the intermediate transferring belt where the black toners are transferred move a long distance to reach a sheet positioned at a downstream-most side of the intermediate transferring belt. Therefore, a time necessary for performing a first printing to the sheet becomes disadvantageously long. For the purpose of dealing with such disadvantage (in other words, for the purpose of speeding up the first printing), the photosensitive drum for black toners is provided on the downstream-most side of the intermediate transferring belt.
Japanese Laid-open Patent Publication No. 2005-234229 (hereinafter, referred to as patent document 1) discloses a measure, which is applied to a tandem type image forming apparatus, for improving a transfer efficiency of toners transferred from photosensitive drums to an intermediate transferring belt. According to this measure, nip pressures (a pressure which can be acquired by coming in press contact with each other) between the photosensitive drums and the corresponding transferring rollers at the time of the primary transfer can be changed, and the nip pressure on an upstream-most side of the intermediate transferring belt in a rotational direction is set to be maximum whereas the other nip pressures are lowered sequentially towards the downstream side (refer to the paragraph [0067] and FIG. 7 of the patent document 1). It is disclosed that making the nip pressures be lowered sequentially towards the downstream-most side of the intermediate transferring belt in the rotational direction prevents aggregation of toners so that the transfer efficiency improves.
Further, Japanese Laid-open Patent Publication No. 2001-282014 (hereinafter, referred to as patent document 2) and Japanese Laid-open Patent Publication No. 2005-024936 (hereinafter, referred to as patent document 3) disclose measures, which are applied to a tandem type image forming apparatus, for preventing the center omission of a transferred image (an image is transferred in such a manner that its center portion becomes pale and is omitted while edges of the transferred image are clear). According to the measures, a nip pressure of a photosensitive drum (in particular, a photosensitive drum for black toners and provided on a downstream-most side of the intermediate transferring belt) which is likely to cause the center omission is set to be lower than nip pressures of other photosensitive drums. It is disclosed that the center omission which is thought to be caused by toners squeezed out from the center towards the edges of the image due to a high nip pressure can be prevented effectively.
However, in the tandem type image forming apparatus capable of performing a color printing, a factor greatly affecting the toner transfer efficiency and the center omission is not only nip pressures between the photosensitive drums and the intermediate transferring belt but also the surface hardness of the intermediate transferring belt, the difference in linear speed between the peripheral surfaces of the photosensitive drums and the surface of the intermediate transferring belt, the peripheral surface characteristics of the photosensitive drums, the surface characteristics of the intermediate transferring belt, and the characteristics of toners. Thus, in relation to the transfer of toners from the photosensitive drums to the intermediate transferring belt, it is not meaningful to focus on nip pressures between the photosensitive drums and the intermediate transferring belt for prevention of the center omission.
On the other hand, in relation to the transfer quality of toners transferred from the photosensitive drums to the intermediate transferring belt, another factor other than the transfer efficiency and the center omission may be important, such as a so-called “toner dispersion” which causes toner particles to be dispersed around in the periphery of the intermediate transferring belt at the time of transfer so that the surface of the intermediate transferring belt is contaminated with the dispersed toner particles to render the outlines of letters, characters, images, etc printed on the sheet. It is assumed that this toner dispersion occurs since fine toner particles attached to the peripheral surface of the photosensitive drum and having an electric charge of a predetermined polarity are dispersed due to a shock given at the time when the toners are electrically peeled off by a bias of an inversed polarity applied by the transferring roller and are transferred to the surface of the intermediate transferring belt.
Such toner dispersion is not mentioned in the patent documents 1-3. Thus, there is no description regarding a measure taken for eliminating the toner dispersion.